1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) system, and in particular, to an apparatus and method for identifying a Relay Station (RS) or a Base Station (BS) by its orthogonal preamble signal with a low Peak-to-Average Power Ratio (PAPR) in a system having 1-hop RSs or multi-hop RSs. The present invention also relates to an apparatus and method for generating another preamble signal using a preamble signal.
2. Description of the Related Art
Provisioning of services with diverse Quality of Service (QoS) requirements at or above 100 Mbps to users is an active study area for the 4th Generation (4G) communication system. Particularly, active research is being conducted on providing high-speed service by ensuring mobility and QoS to a BWA communication system such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Such major examples are Institute of Electrical and Electronics Engineers (IEEE) 802.16a and IEEE 802.16e.
The IEEE 802.16a and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) for physical channels in order to support a broadband transmission network. IEEE 802.16a considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SS's mobility to the IEEE 802.16d communication system. Hereinafter, a mobile SS will be referred to as an MS.
In general, since a BS and an MS communicate with each other via a direct link, a highly reliable radio link can easily be established between them in the IEEE 802.16e communication system. However, due to the fixedness of the BSs, the configuration of a wireless network is not flexible, making it difficult to provide an efficient service in a radio environment experiencing a fluctuating traffic distribution and a substantial change in the number of required calls. The above drawback can be overcome by a relay service that delivers data over multiple hops using MSs or fixed or mobile RSs. The use of the multi-hop relay scheme expands cell coverage.
FIG. 1 illustrates the configuration of a conventional BWA communication system using RSs.
Referring to FIG. 1, the BWA communication system is configured in a multi-cell structure. Specifically, it includes cells 100 and 140, BSs 110 and 150 for managing the respective cells 100 and 140, a plurality of MSs 111, 113, 151, 153 and 155 within the cells 100 and 140, a plurality of MSs 121, 123, 161 and 163 under management of the BSs 110 and 150 but in areas 130 and 170 outside the cells 100 and 140, and RSs 120 and 160 for providing relay paths between the BSs 110 and 150 and the MSs 121, 123, 161 and 163 in the areas 130 and 170. Signaling is carried out in OFDM/OFDMA between the BSs 110 and 150 and the MSs 111, 113, 121, 123, 151, 153, 155, 161 and 163. Although the MSs 111 and 113 and the RS 120 within the cell 100 can communicate directly with the BS 110, the MSs 121 and 123 in the area 130 cannot communicate directly with the BS 110. Therefore, the RS 120 covers the area 130 and relays signals between the BS 110 and the MSs 121 and 123. In other words, the MSs 121 and 123 can send and receive signals to and from the BS 110 via the RS 120.
Although the MSs 151, 153 and 155 and the RS 160 within the cell 140 can communicate directly with the BS 150, the MSs 161 and 163 in the area 170 cannot communicate directly with the BS 150. Therefore, the RS 160 covers the area 170 and relays signals between the BS 150 and the MSs 161 and 163. In other words, the MSs 161 and 163 can send and receive signals to and from the BS 150 via the RS 160.
In the OFDM communication system, an MS acquires frequency synchronization and frame synchronization from a BS and an RS and identifies them by their preamble signals.
However, since data is sent on a plurality of subcarriers, the amplitude of a final OFDM signal can be expressed as the sum of the amplitudes of the subcarriers. If the subcarriers are at the same phase, the OFDM signal has a very high PAPR. In general, the signal with the very high PAPR is beyond the linear operation range of an amplifier and thus experiences distortion after passing through the amplifier. As a result, the high-PAPR signal not only decreases the efficiency of a linear amplifier but also makes the operation point of a non-linear amplifier enter into a non-linear range, thereby causing inter-modulation and out-of-band spectrum radiation.
In contrast, a low-PAPR preamble sequence prevents non-linear distortion in a High Power Amplifier (HPA), thus leading to high-quality signal transmission and highly efficient power amplification. A typical OFDM communication system generates a plurality of low-PAPR preamble sequences, but the number of low-PAPR preamble sequences is limited.
Accordingly, there exists a need for an apparatus and method for generating a low-PAPR preamble sequence with orthogonality and generating an additional low-PAPR preamble sequence using the low-PAPR preamble sequence, for use in a BS and an RS.